To obtain further information on the nature and significance of the effects of length changes on the activity of muscle the following types of experiments are planned. While in our previous work these effects usually were produced by force steps during isotonic contraction, also length steps will be used. This procedure may give a more quantitative picture of the magnitude of the responses in different phases of a twitch. In an earlier study an apparent contradiction was found when the effect of length changes during contractures was recorded under isometric or isotonic conditions. A solution of this problem may be found by using the improved technique with better time resolution now available. The control of the rate of relaxation will be studied. To find an explanation for the large difference in the time course of relaxation between isotonic and isometric twitches, the effect of mechanical interventions, of ions and drugs will be studied. A point of special interest is the fact that in skeletal muscle the last part of relaxation in a freeloaded twitch is extremely slow, lasting for more than a minute (at O degrees). It is also planned to study the effect of length changes during twitches of smooth muscles.